1. Field of the Invention
The invention relates to optically linked data processing systems and, more particularly, to optical star couplers for coupling optical transmission lines and a plurality of stations in such systems.
2. Background Art
Fiber-optic transmission of data offers many advantages over the conventional form of data transmission in data processing systems. Optical signals are generally immune to errors caused by electromagnetic interference and radio frequency interference, and these systems do not spark or short circuit. Fiber-optic transmission also eliminates ground loop problems by providing electrical isolation between optically linked equipment.
A fiber-optic system for data transmission may be employed in a distributor processing system, in a local area network, in a data bus system and the like. These systems frequently require the use of a plurality of processing stations or terminals to communicate with each other as well as with peripheral equipment. Fiber-optic transmission systems typically employ a ring or a star type architecture.
A ring type architecture generally uses Tee type couplers. A ring type architecture using Tee couplers is disclosed in the U.S. Pat. No. 4,072,399. Typically, in a ring type system, the light or the optical signal from one terminal is routed to the next terminal in the ring, where it is tapped off (received) and retransmitted. When an optical signal is tapped off, it reduces the signal level of the remaining signal being transmitted to the next terminal in the ring. In such an arrangement, there can be a substantial difference in the signal strength between a near terminal and a far terminal. This disparity in the signal strength can be avoided by each successive terminal receiving the signal, combining it with its own output and then retransmitting the new signal. One drawback of such a system is that the system fails when any terminal loses power, or when any cable breaks, or if any terminal is disconnected from the ring architecture. Because of these and other reasons, most fiber-optic data transmission systems use a star coupler architecture.
There are two types of star couplers; reflective type and transmissive type. A star coupler has a plurality of input ports for receiving optical signals and a plurality of output ports for transmitting optical signals. The reflective star coupler combines the optical signals in a mixing section. The mixing section is terminated with a mirror which reflects the optical signals back into cables that first brought the signal to the mixing section. The transmissive optical couplers receive the signals at one set of ports, combine these signals and then distribute a portion of the mixed signals to each of its output ports. Both types of star couplers work equally well backwards, i.e. the light entering the output ports will be distributed to the input ports. But for clarity, the ports are generally designated as input ports or output ports. The present invention applies to the transmissive type star couplers.
Although the figures in this application show the star coupler with inputs on one side and outputs on the other, such an arrangement is shown only for clarity and simplicity. As noted earlier, input ports and output ports are interchangeable. This arrangement may not be the best way to package the device. It is easier to control bends inside the coupler package than outside it. Therefore, the primary consideration should be to provide the best situation for the external optical fiber connection. In most applications, two fibers from each terminal are connected to the star coupler. These fibers should be kept together to the maximum extent possible instead of splitting them to go to different sides of the star coupler. Therefore, it may be desirable to have all fibers go to the coupler through one connector, which has fiber-optic pins or sockets. It is generally desirable to make the pin assignments (geometric configuration) in the star coupler such that the repeater circuits are separated from the terminals. In other words, the fibers going to the repeaters and the fibers going to the terminals go off in different directions. Additionally, a minimum separation of pairs of fibers from each other should be provided, and each pair should have a standardized color code -- one color for high level (transmitter output) and the other color for the low level (star coupler to the repeater).
In a system configuration using star couplers, each star coupler receives the transmitted signal and divides the signal evenly to all the receiving terminals, thereby minimizing the differences in the signal levels between near and far terminals. However, this division of optical signal proportionally reduces the level of signal received by each terminal. As an example, if a star coupler receives a signal of level "Y" and transmits it to 24 terminals, then each terminal will receive a signal of level Y divided by 24. Thus, for a large system there is not much signal left for any terminal. The diminution of an optical signal because of its division in the star coupler is generally referred to as a "fan-out" or a "furcation" loss. Thus, the use of fiber-optics to interconnect a local area network or a data base transmission system is generally limited by these furcation losses. The present limitation is the use of approximately 64 terminals for each star coupler. Additionally, for large systems the terminals must be designed for a low signal level which is highly undesirable.
When a large number of outputs are required or inputs are clustered in several locations, it would be desirable to use a plurality of couplers cascaded with repeater amplifiers. However, when conventional star couplers are cascaded in this way, as shown in FIG. 1, the optical signal forms a continuous loop. This continuous loop phenomenon is called a "lockup". The lockup occurs because light output from the first star coupler is amplified by the repeater amplifier and sent to the second star coupler, where it is divided and a part of the signal is amplified again by the repeater amplifier and returned to the first star coupler, thereby constituting a loop. This problem has been avoided in the past by using only one star coupler in a system with enough ports for all terminals.
The present invention addresses the problem by providing a star coupler which when cascaded with repeater amplifiers does not lockup, while at the same time preserving the advantage of equal signal strength and the ability to monitor the system's integrity by monitoring its own signal returning from the star coupler.